Stop building libcxx with LIBCXX_ABI_UNSTABLE=TRUE

This didn't seem to actually be necessary, not even in the initial
commit adding support for building a DLL of libcxx. It might have
been necessary at some earlier point while working on building a DLL.

README.md

@@ -71,9 +71,6 @@ normal GCC/binutils based MinGW.
 
 - The windres replacement, llvm-rc, isn't very mature and doesn't support
   everything that GNU windres does.
-- The DLL version of libc++ doesn't have a fixed supported ABI, so in theory,
-  code built against an older version of the DLL is not guaranteed to work
-  with a newer version of the DLL.
 - The toolchain defaults to using the Universal CRT (which is only available
   out of the box since Windows 10, but can be installed on Vista or newer)
   and defaults to targeting Vista. These defaults can be changed in